What factors drive population growth rates? One major indicator and determinant of demographic change is fertility, which demographers express as the total fertility rate, the number of births that can be expected to occur to a typical woman in a given society during her childbearing years. Fertility is a function of a woman's fecundity (her physiological ability to conceive and bear children (footnote 4)) and of social, cultural, economic, and health factors that influence reproductive choices in the country in question. The most important non-physical factors influencing a country's total fertility rate include relationship status (the fraction of women who are married or in a relationship that exposes them to the possibility of becoming pregnant); use of contraception; the fraction of women who are infecund—for example, because they are breastfeeding a child; and the prevalence of induced abortion (footnote 5).

Fertility levels are lower in developed countries than in developing nations because more women in developed countries work outside of the home and tend to marry later and to use contraception and abortion to delay or prevent childbearing. Nevertheless, fertility rates in nearly all countries have been falling since the 1950s (Fig. 4). Most of the exceptions are in Central and Western Africa.

Fertility patterns can vary widely within countries. Racial and ethnic minorities may have higher fertility rates than the majority, and families with low incomes or low levels of education typically have more children than those that are affluent or well-educated. Women who work outside the home generally have fewer children than those who stay home, and rural families have more children than city dwellers. In 2006, the number of births per 1,000 people worldwide averaged 21, with extremes ranging from a low of 8 or 9 (mainly in northern and western Europe and some former Soviet republics) to 50 or more in a few west African nations (footnote 6).

Mortality is the second major variable that shapes population trends. A population's age structure is an important factor influencing its death rate. Death rates are highest among infants, young children, and the elderly, so societies with many elderly people are likely to have more deaths per 1,000 people than those where most citizens are young adults. Developed countries with good medical services have more people in older age brackets than developing countries, so the developed societies can have higher death rates even though they are healthier places to live overall.

To assess longevity in a society, demographers calculate life expectancy—the age that a newborn would, on average, live to, assuming she were subject to a particular set of age-specific mortality rates—usually those prevailing in a particular year. The probability that a child will die at a given age drops through childhood and adolescence after she passes through the vulnerable early years, then starts to rise gradually in mid-life. Figure 5 shows remaining life expectancy at birth, 65 years of age, and 75 years of age in the year 2000 for people in the United States. Americans who were age 65 or 75 by 2000 had already survived many common causes of death, so they could expect at that point to live to an older age than would a baby born in that year (if life expectancy did not change during the baby's lifetime).

Figure 5. Life expectancy at birth, age 65, and age 75, United States, 2004See larger image

Source: Courtesy National Center for Health Statistics, U.S. Health with Chartbook on Trends in the Health of Americans. 2006.

Life expectancy is trending upward around the world, but a substantial gap remains between developing and developed countries (Fig. 6). In 2006, life expectancies at birth ranged from the mid-30s in some African countries to the high 70s or low 80s in the United States, Australia, Japan, and some European countries (footnote 7).

What factors raise life expectancy? Because of the way in which it is calculated, life expectancy serves as a measure of the general health of the population, which depends on the satisfaction of many basic human needs such as adequate nutrition, clean water and sanitation, as well as access to medical services like vaccinations. Addressing these requirements reduces the incidence of many preventable illnesses. For example, nutritional deficiencies cause common illnesses like scurvy and pellagra, while dirty water and poor sanitation spread infectious agents such as cholera and typhoid. (For more details, see Unit 6, "Risk, Exposure, and Health," and Unit 8, "Water Resources.")

New threats to health are continually emerging, and often are spread across international borders through trade and human or animal migration. Recent examples that are severe enough to affect life expectancy in large areas include the HIV/AIDS pandemic and potentially avian flu and multi-drug-resistant malaria and tuberculosis. Researchers are also gaining new insight into existing threats, such as indoor air pollution from combustion of primitive biomass fuels like crop waste and dung. Exposure to these pollutants is a major factor contributing to infant mortality and lower life expectancy in developing countries (Fig. 7). Environmental investments, such as providing cleaner energy sources and upgrading sewage treatment systems, can significantly improve public health.

Another step that increases life expectancy is creating a public health infrastructure that can identify and respond quickly to disease outbreaks, famines, and other threats. When severe acute respiratory syndrome (SARS) emerged as a disease that might cause an international epidemic, the U.S. Centers for Disease Control and Prevention (CDC) launched an emergency response program that required health departments to report suspect cases to CDC for evaluation, developed tests to identify the SARS virus, and kept health care providers and the public informed about the status of the outbreak. The United States and many other countries also reported their SARS cases to the World Health Organization. These types of close surveillance and preventive steps to control infections can help prevent diseases from spreading widely.

The third major factor that drives population trends is migration, which includes geographic population shifts within nations and across borders. Migration is less predictable over long periods than fertility or mortality, since it can happen in sudden waves—for example, when refugees flee a war—or slowly over many years. Immigration often changes host nations' or regions' ethnic mixes and strains social services. On the positive side, it can provide needed labor (both skilled and unskilled). For source countries, however, immigration may drain away valuable talent, especially since educated and motivated people are most likely to migrate in search of opportunities.